NXP backend: Resolve mean.dim format related issue.

[MartinPavella]

Summary

The existing mean.dim support in NXP backend failed to consider the format related implications of the operators
dimension reducing ability". This PR correctly identifies these edge cases and handles conversion to Neutron IR accordingly.

Test plan

Unit tests provided.

cc @robert-kalmar @JakeStevens @digantdesai @rascani

[pytorch-bot]

🔗 Helpful Links

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❌ 1 New Failure, 1 Unclassified Failure

As of commit 76d19fb with merge base ce2f3f9 ():

NEW FAILURE - The following job has failed:

• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 9d318670f5249d22a1db8e52311bd3dabfaa3b9989dbea98ae2c413e5a048b61 /exec failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• pull / test-qnn-direct-build-linux / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command docker exec -t 62979def541ad9a51bffae7d96daa4942c4fe365234553ce7a63419a0f4bbfae /exec failed with exit code 127

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[MartinPavella]

@novak-vaclav please feel free to have a look.